Mitosis is a key stage during the life of a cell. It is the stage where a bipolar spindle structure is organized to segregate duplicated chromosomes into the two daughter cells. Spindle organization and function require exquisite precision, robustness and fidelity. Defects associated with the spindle can lead to defects in chromosomal segregation, or aneuploidy, which has been correlated with some types of cancer. The spindle is a macromolecular machine made of microtubules, microtubule-associated proteins (MAPs), molecular motors and other regulatory proteins. Of intense interest have been molecular motors, which perform work such as cross-linking and sliding microtubules apart to form the bipolar spindle, or to depolymerize microtubules to maintain proper spindle lengths, or to carry chromosomes to opposite spindle poles. Surprisingly, while we have learned much about motors involved in mitosis, we still know very little about the MAPs and other regulatory proteins and how they coordinate with motors to bring about proper spindle formation. My laboratory uses the relatively simple fission yeast Schizosaccharomyces pombe and human cultured cells to address conserved mechanisms of spindle organization and function. This particular project focuses on how the initial bipolar spindle is formed at the start of mitosis, the stage termed prophase. We focus on the MAPs that contribute to spindle formation. Using fission yeast as a gene discovery tool, we have begun to define the roles of a new gene we called psr1+ (poles separation regulator 1). Our work indicates that psr1p organizes the initial bipolar spindle during prophase. Psr1-deletion leads to high frequency of monopolar spindles and subsequent chromosome segregation defects. Fission yeast psr1+ appears to have a human functional homolog. We have begun to characterize a novel human gene we called PSR1. In HeLa cells, siRNA of PSR1 also leads to high frequency of monopolar spindles and subsequent chromosome segregation defects. This proposal aims to combine modern cell and molecular biology techniques in fission yeast and human cultured cells, biochemistry, high-resolution optical live-cell imaging, and innovative microfluidic techniques to control cellular microenvironment, to reach a mechanistic understanding of bipolar spindle formation.